The Sewer Demon: The Roman Mystery Scrolls 1

At the end of book 17 of the Roman Mysteries, Caroline Lawrence introduced us to Threptus - a young beggar boy who is inspired to learn to read, write and lead an honest life by Lupus's example. Threptus needs to earn money to be schooled and as he seeks employment he meets many fascinating residents of the Port of Ostia ... all of whom have a mystery which needs to be solved. Each of the Threptus books will be a mini mystery.